[Python-ideas] template strings

[Eric V. Smith]

On 08/17/2015 04:13 PM, Yury Selivanov wrote:
> In ECMAScript 6 there is a concept of Template Strings [1]. What if we add
> something similar in Python?
> 
> Some key ideas
> --------------
> 
> 1. Template Strings (TS) will be built on top of PEP 498 machinery (if
> accepted).
> 
> 2. The syntax will match the following:
> 
>     {python identifier}{optional whitespace}{string literal}
> 
> where "python identifier" can be any valid python name *except* r, u, b,
> or f.
> 
> Some examples of valid TS:
> 
>    ##
>    _'foo {bar}'
> 
>    ##
>    sql = db.escape
>    sql """
>       SELECT ... FROM ...
>    """
> 
>    ##
>    from framework import html
>    html"""
>       <div class="caption">
>         {caption}
>       </div>
>    """
> 
> 3. A special magic method will be added: __format_str__(string,
> values_map).
> 
> For instance,
> 
>     b = 10
>     print(_'something { b+1 }')
> 
> will be equivalent to
> 
>     b = 10
>     print(_.__format_str__('something { b+1 }', {' b+1 ': b + 1}))
> 
> (or however PEP 498 will be implemented).

I'm not convinced this is a great idea. It's almost like it wants to be
a way to pass in ASTs to functions, but only for the limited case of
f-string expressions. Maybe that's as far as we'll ever want to go, and
this is good enough.

As you say, it would allow easy implementation of i18n on top of PEP 498.

If we do go this route, we should reserve some names for Python's own
use in the lexer. For example, if this proposal were in place before we
added bytes strings, there would be no easy way to add them.

Eric.